Coolant mixing grid for nuclear fuel assembly

ABSTRACT

A fuel assembly for a nuclear reactor is described, the fuel assembly including: a plurality of fuel pins ( 12 ) extending substantially parallel to the axis of the assembly and to each other; at least two structural grids spaced apart from each other, the grids being in contact with said fuel pins ( 12 ) and maintaining said fuel pins substantially mutually parallel and preventing contact therebetween, wherein the fuel assembly further comprises at least one mixing grid ( 50 ) situated intermediate said at least two structural grids, the fuel assembly being characterized in that said mixing grid ( 50 ) is positioned and fixedly located out of substantial contact with said fuel pins ( 12 ), the mixing grid also having turbulence inducing means ( 61 ) to promote turbulence in a coolant ( 62 ) flowing through said fuel assembly in use and in that the mixing grid is formed from sheet metal wherein the plane of the metal sheet from which the mixing grid is formed lies in a plane which is transverse to the axis of the fuel pin assembly.

The present invention relates to fuel rod assemblies for nuclearreactors.

Fuel rod assemblies, or “elements”, for nuclear reactors comprise aplurality of parallel fuel rods or pins which are maintained a setdistance apart from each other and mutually parallel by grids at thetop, bottom and usually at one or more intermediate positionstherebetween in the fuel assembly. A further function of the grids is tomaintain the fuel pins apart and thereby to prevent fretting of the fuelpins leading to mechanical damage.

Grids employed to the present time are generally constructed by weldingtogether for example many individual components of sheet material toform an array of individual apertures each receiving a fuel pin andhaving resilient locating means such as “spring fingers” for exampleformed from the sheet material within the apertures so as to locate thefuel pin as centrally as possible within the aperture. Such gridsgenerally also have vanes integrally formed from the sheet material toinduce turbulence in the gas or liquid coolant which flows through thefuel assemblies in a direction generally parallel to the axis of thefuel assembly. The purpose of inducing turbulence in the coolant is toimprove the heat extraction from the fuel pins by improved coolantmixing and thus to prevent overheating thereof.

Grids such as those described above are generally fabricated from sheetmaterial wherein the plane of the sheet material is generally parallelto the axis of the fuel assembly and comprise substantial quantities ofmetal in their construction which is detrimental in that the metal isparasitic in absorbing neutrons from the fuel and reducing power outputfrom the reactor. A typical example of such a mixing grid is describedin U.S. Pat. No. 5,183 629.

In a pressurised water reactor (PWR) for example, the individual fuelpins may be about 4 m in length. The fuel pin comprises an outer tubularsheath known as the “cladding” made from a metal alloy such as“Zircaloy”™ for example, within which cladding is the fuel per se. Owingto the high temperature reached by the fuel pin in operation, the outersurface of the cladding is subject to corrosion and oxidation where itis in contact with the coolant. The maximum depth of corrosion orthickness of oxide corrosion product of the cladding occurs at about 80%up the length of the pin from the point of entry of the coolant into thefuel pin assembly. The maximum allowable thickness of the oxide layer isa potential life-limiting factor of the fuel pin and consequently of thefuel assembly. Therefore, it is desirable to improve the cooling of thecladding in at least this region so as to reduce the rate of corrosionof the cladding.

An improvement of the cooling of the cladding to reduce the rate ofcorrosion also may have the effect of delaying or providing a greatersafety margin prior to the onset of departure from nucleate boiling(DNB) and allowing the fuel assembly to be operated at a higher powerlevel than would otherwise be possible. Nucleate boiling is the mostefficient form of heat extraction. DNB occurs where a film of steamoccurs at the surface of the fuel pin and heat transfer from the pin tothe coolant decreases dramatically resulting in failure of the pinwithin a very short time.

According to a first aspect of the present invention, there is provideda fuel assembly for a nuclear reactor, the fuel assembly including: aplurality of fuel pins extending substantially parallel to the axis ofthe assembly and to each other; at least two structural grids spacedapart from each other, the grids being in contact with said fuel pinsand maintaining said fuel pins substantially mutually parallel andpreventing contact therebetween, wherein the fuel assembly furthercomprises at least one mixing grid situated intermediate said at leasttwo structural grids, the fuel assembly being characterised in that saidmixing grid is positioned and fixedly located out of substantial contactwith said fuel pins, the mixing grid also having turbulence inducingmeans to promote turbulence in a coolant flowing through said fuelassembly in use and in that the mixing grid is formed from sheet metalwherein the plane of the metal sheet from which the mixing grid isformed lies in a plane which is transverse to the axis of the fuel pinassembly.

The mixing grid of the fuel assembly according to the present inventionmay alternatively be formed from wire for example and being joined bywelding for example at positions where wires cross and also havingturbulence inducing means such as vanes for example attached to thewires.

However, in a preferred embodiment of the fuel assembly of the presentinvention, the mixing grid may be formed from sheet metal by for examplepressing or stamping wherein the plane of the metal sheet from which themixing grid is initially formed lies in a plane which is transverse tothe axis of the fuel pin assembly.

The thickness of the sheet material may be in the range from 0.5 mm toabout 1 mm for example. However, the thickness is not considered to becritical and may be chosen so as to be resistant to forces imposed bythe coolant flow whilst allowing easy mechanical forming thereof.

The mixing grid may be in the form of a framework having an array ofapertures of predetermined size and shape, such as square, triangular orhexagonal for example, and through which the fuel pins extend,preferably without making any significant contact with the surroundingframework of each aperture.

Where formed from sheet metal as described above by pressing orstamping, turbulence inducing means such as vanes may be integrallyformed during such a forming operation and deformed away from a positionlying in the sheet plane to a desired angle so as to provide the optimumturbulence inducing effect. Such turbulence inducing means may be formedon an inner edge or edges of each or any apertures as desired consistentwith producing the optimum desired turbulence. In view of the manydifferent designs of fuel pin assembly in existence, the optimumconfiguration and distribution of turbulence inducing means may bedetermined by experimentation.

Some or all of the framework members surrounding each aperture may betwisted about the plane of the sheet so as to form turbulence inducingfeatures per se. Such twisting may also reduce the pressure increasenecessary to pump coolant through the fuel pin assembly.

The overall outer boundary shape of the mixing grid will correspond tothe particular fuel pin assembly into which it is being assembled andmay for example be square or hexagonal.

The mixing grid of the fuel pin assembly according to the presentinvention may not extend to and encompass the outer peripheral ring offull pins. The reason for this is that the fuel pins in the outerperipheral ring tend to run cooler than inner fuel pins and thus, thedegree of corrosion is less under normal operating conditions. A furtheradvantage of the mixing grid not extending to the outer ring of fuelpins is that the risk of snagging of the fuel assembly during insertioninto and removal from the reactor core is lessened.

The mixing grid of the fuel pin assembly of the present invention may beheld in position within the fuel pin assembly by, for example, so-calledthimble tubes in which reactor control rods run; the appropriate gridapertures being sized so as to be located by welding or swaging theretofor example. Alternatively, in a preferred embodiment of the presentinvention, short stub tubes may be fixed to the mixing grid by welding,for example, at positions which correspond to some or all of the thimbletubes such that the short tubes fit over the thimble tubes and are inturn fixed, by crimping or welding for example, to the thimble tubes.

Determination of the optimum position or positions of mixing gridswithin the fuel pin assembly may be determined by experimentation andwill vary according to the fuel pin assembly design, the type of coolantand/or the type of reactor in question. Sufficient mixing grids may beemployed such that the maximum temperature of the hottest fuel pin orpins are reduced to a level where cladding corrosion does not restrictthe burn-up life of the fuel.

Improved mixing of the flowing coolant by inducing turbulence thereinappears to occur in known fuel pin assemblies in the regions immediatelypreceding and immediately following a structural grid. Therefore, itmight be thought that the inclusion of known structural grids inadditional positions within the fuel assembly would be advantageous inpromoting extra turbulence to improve cooling and heat extraction indesired locations so as to reduce corrosion/oxidation rate. However,this solution has several significant disadvantages. Firstly, prior artstructural grids are extremely expensive to produce owing to theircomplex structure. Secondly, known structural grids contain substantialquantities of metal which absorbs neutrons causing significant parasiticpower loss. Thirdly, additional structural grids provide additionallocations where grid-to-rod fretting damage can occur. The mixing gridof the fuel pin assembly of the present invention on the other hand issimple and cheap to make as it may be formed from a single stamping orpressing of an initially flat metal sheet; it contains only a relativelysmall quantity of metal so that parasitic losses due to neutron captureare minimal; and, the fuel pins of the preferred embodiment do not touchthe mixing grid and therefore there can be no fretting damage to thefuel pin caused by the mixing grid.

According to a second aspect of the present invention, there is provideda mixing grid for a fuel pin assembly according to the first aspect.

In order that the present invention may be more fully understood,examples will now be described by way of illustration only withreference to the accompanying drawings, of which:

FIG. 1 shows a perspective view of part of a prior art fuel pin assemblyfor a PWR nuclear reactor;

FIG. 2 shows a graph of average oxide layer thickness vs. axial positionalong a fuel rod from the coolant entry point;

FIG. 3 shows a perspective view of a section of a fuel pin assemblyaccording to a first embodiment of the present invention;

FIG. 4 shows a plan view of the embodiment of FIG. 3;

FIG. 5 shows a perspective view of a modified embodiment similar to thatshown in FIG. 3;

FIG. 6 shows a plan view of a small section of a mixing grid; and

FIG. 7 which shows a cross section on the line 7-7 of FIG. 6.

Referring now to the drawings and where FIG. 1 shows a perspective viewof a known fuel pin assembly 10, also called a fuel “element”, for a PWRnuclear reactor core (not shown) and FIG. 2 shows a graph of averageoxide layer thickness on fuel pin cladding. The assembly 10 comprises aplurality of fuel pins 12 arranged in a square array; top 14 and bottom16 nozzles for locating the assembly in the reactor core; and,structural spacer grids 18 for aligning the pins 12 parallel to eachother and to the assembly axis. The assembly shown in FIG. 1 is about 4m in length, intervening parts 20 of the fuel pin length having beenremoved in the view shown. The structural spacer grids 18 have aplurality of apertures, each one accepting a fuel pin 12 or a thimbletube 22 (see FIGS. 3, 4 and 5) as appropriate and have resilient springfingers (not able to be seen) in the apertures to grip the fuel pin toprevent any relative movement and consequent fretting damage: Inoperation, coolant water is pumped upwardly from the bottom nozzle 16through the assembly 10 to exit via the top nozzle 14. Turbulenceinducing vanes (not able to be seen) are provided on the grids 18 topromote mixing of the coolant and hence improve cooling of the fuel pins12.

The graph of FIG. 2 does not relate to the specific fuel assembly 10 ofFIG. 1 which is merely exemplary of fuel pin assemblies. FIG. 2 shows acurve 30 showing average oxide layer thickness on the cladding againstaxial position on the fuel pin 12. In the curve 30, maxima 32 and minima34 (only one of each indicated by arrows for the sake of clarity) areshown at various positions along the fuel pin length. The fuel pinassembly corresponding to the curve 30 has structural spacer gridslocated at positions which lie between a maxima and the precedingminima, e.g. at position 36 and other corresponding positions. Thegreatest average oxide layer thickness occurs at the maxima 38 andrepresents an oxide thickness which is effectively life-limiting for thefuel pin. The ability to reduce the oxide thickness at this position,and also possibly at the preceding maxima 40, would enable the fullburn-up potential of the fuel per se to be utilised or would allow thefuel assembly to operate at a higher power level or a combination ofthese advantages.

In the fuel pin assembly according to the present invention there isprovided an additional grid 50 as shown in FIGS. 3 and 4, of which FIG.3 shows a perspective view of part of the area of a short axial portionof a fuel pin assembly and FIG. 4 a plan view thereof. A plurality offuel pins 12 are again shown, the fuel pin comprising an outer tubularsheath or cladding 22 which is filled with fuel material (not shown).Interspersed amongst the fuel pins 12 are thimble tubes 52 which receivecontrol rods (not shown) to control the power output of the reactor. Thegrid 50 is stamped from sheet material such as Zircaloy (trade mark) andcomprises a continuous framework 56 surrounding apertures 58 throughwhich both the fuel pins 12 and thimble tubes 52 extend. However, thefuel pins 12 do not touch the framework 56 whereas the thimble tubes 52are swaged outwardly at the location 60 where they pass through theapertures 58 so as to fixedly grip the framework 56 and thus locate thegrid 50 in a desired position and orientation relative to the fuel pins12. During the stamping operation, turbulence inducing means 61 in theform of vanes are integrally formed with the framework 56, the vanesbeing deflected away from the plane of the original sheet materialduring the forming operation to a desired configuration so as tooptimise turbulence in the coolant water, indicated only schematicallyby the arrows 62, flowing through the fuel pin assembly 10. The grid 50extends only up to the outer ring 64 of fuel pins since the outer ringof pins run at a lower temperature and have a consequently thinner oxidethickness layer. The grid 50 is positioned intermediate adjacent spacergrids 18 in the vicinity of the maxima 38 (and also possibly in thevicinity of maxima 40). The increased turbulence and improved coolantmixing caused by the grid 50 lowers the temperature of the fuel pins atthis point and consequently also reduces the oxide thickness. The lowvolume or mass of the mixing grid 50 does not significantly increase theparasitic loss due to neutron capture.

FIG. 5 shows a slightly modified grid 50 having turbulence inducingvanes 70 of different form. Other features remain essentially the sameas in FIGS. 2 and 3.

FIGS. 6 and 7 show a small part of the framework 56 of a grid 50. Inthis modification, some or all of the individual framework members 80are twisted out of the plane of the original sheet from which the gridis stamped or pressed to an angle so as to promote turbulence in thecoolant thereby. Vanes as described above with reference to FIGS. 3, 4and 5 may or may not be employed depending upon the specific geometryand requirements of the fuel assembly 10 in question.

What is claimed is:
 1. A fuel assembly for a nuclear reactor including:a plurality of fuel pins having respective axes extending substantiallyparallel to each other; at least two structural grids spaced apart fromeach other, the structural grids being in contact with said fuel pinsand maintaining said fuel pins substantially mutually parallel andpreventing contact therebetween, wherein the fuel assembly furthercomprises at least one mixing grid situated intermediate said at leasttwo structural grids, said mixing grid having turbulence inducing meansto promote turbulence in a coolant flowing through said fuel assembly inuse, wherein said mixing grid is positioned and fixedly located out ofcontact with said fuel pins and wherein the mixing grid is formed from asingle sheet of metal wherein the plane of the metal sheet from whichthe mixing grid is formed lies in a plane which is transverse to theaxes of the fuel pins.
 2. A fuel assembly according to claim 1, whereinthe thickness of the sheet material is lies in the range from 0.5 mm toabout 1 mm.
 3. A fuel assembly according to claim 1 wherein the mixinggrid is in the form of a framework having an array of apertures ofpredetermined size and shape.
 4. A fuel assembly according claim 1wherein the turbulence inducing means include vanes attached to themixing grid.
 5. A fuel assembly according to claim 1 wherein the vanesare integrally formed with the mixing grid.
 6. A fuel assembly accordingto claim 3 wherein the framework includes framework members surroundingeach aperture and wherein at least some of the framework memberssurrounding each aperture are twisted out of the plane of the sheet soas to form turbulence inducing features.
 7. A fuel assembly according toclaim 1 wherein the mixing grid does not extend to and encompass anouter peripheral ring of fuel pins.
 8. A fuel assembly according toclaim 1 wherein the mixing grid is located and held in position withinthe fuel pin assembly by thimble tubes in which moderator control rodsrun.
 9. A fuel assembly according to claim 8 wherein the mixing grid islocated by swaging to the thimble tubes.
 10. A fuel assembly accordingto claim 8 wherein the mixing grid is first joined to short tubes whichare fitted over the thimble tubes and are fixed to the thimble tubes.11. A fuel assembly according to claim 1 wherein the mixing grid islocated at a position between two adjacent structural grids where,during operation of the reactor in use, the hottest region of the fuelpins is located.
 12. A mixing grid for use in a fuel pin assemblyincluding a plurality of fuel pins having respective axes extendingsubstantially parallel to each other, at least two structural gridsspaced apart from each other, the structural grids being in contact withthe fuel pins and maintaining the fuel pins substantially mutuallyparallel and preventing contact therebetween, said mixing grid includingturbulence inducing means to promote turbulence in a coolant flowingthrough the fuel assembly in use and wherein: said mixing grid is formedfrom a single sheet of metal; said mixing grid is sized and configuredto be situated intermediate said at least two structural grids,positioned and fixedly located out of contact with said fuel pins, andsuch that, when said mixing grid is so positioned, the plane of themetal sheet from which the mixing grid is formed lies in a plane whichis transverse to the axes of the fuel pins.
 13. A fuel assemblyaccording to claim 8 wherein the mixing grid is located by welding tothe thimble tube.